A porous resin film has been conventionally used as a filtration material for water purifiers, air purifiers and the like, clothing, synthetic paper, sanitary materials, medical materials, construction materials, electronic materials, air permeation sheets for agriculture, optical light reflectors for liquid crystal display, solar power generator, and the like, separators for batteries, separators for electrolytic capacitors and the like, and it has been desired to further improve and develop such a resin film for making the film thin and the durability in all of these applications.
For example, a lithium secondary battery, which has been used in electric instruments or mobile instruments such as cellular phones and notebook computers, has a high energy density, and therefore the lithium secondary battery have been increasingly expanded in production and consumption since it was commercially made available in the beginning of 1990s. Further, in recent days, studies are made to use the lithium secondary battery not only as a main power source for the people's livelihood, but also as a substitute for a lead battery installed in a vehicle, and therefore it is now required more than ever to have improved properties and safety. As such, physical properties of a separator made of a porous resin film are also required to have better performances like ion permeability and strength, and higher safety.
For example, a method was proposed for obtaining a porous film by stretching a film-like material comprising a resin composition containing a thermoplastic resin and a filler in TD direction by using a stretching device having a pre-heating part, a stretching part, and a heat treatment part (Patent Document 1).
On the other hand, in the field of light reflector, although a polyolefin film is used for a monitor of a personal computer, a thin-screen liquid crystalline television, a cellular phone, and a portable game machine, it is being attempted to have a small weight and a thin film and also a polyolefin film having higher stability with time and light reflection rate is needed.
In this regard, a reflector comprising a laminate made of a porous resin sheet consisting of a polyolefin resin and an inorganic filler, and a support is suggested (Patent Document 2).
Further, an inorganic filler in which the surface of inorganic particles is treated with a surface active agent (A) and a compound (B) having an ability to chelate alkaline metals, and a resin composition added with the filler are suggested (Patent Document 3).